Ever since radio transmission first became a reality, utilization of the electromagnetic (EM) spectrum for communication has continued to grow. Every part of the electromagnetic spectrum from AM radio up through infrared and visible light is now used to transmit information. Modern consumer devices which transmit and/or receive EM signals include FM and AM radios, CB and personal radios, televisions, pagers, cell phones, remote controls for consumer electronics, GPS receivers, PDAs, cordless phones, wireless local area networks, wireless computer peripherals, garage door openers, wireless door bells, wireless home and car burglar alarm components, etc. New low-power EM communication standards such as Bluetooth are resulting in a new generation of consumer electronics such as video cameras, VCRs and the like which can all communicate wirelessly.
In an attempt to minimize interference between wireless devices, while having as many devices make use of the available EM spectrum as possible, countries such as the United States have enacted complex laws and regulations specifying the types of use for different portions of the EM spectrum (including geographic and power limitations), and in some cases requiring licensing of classes of transmitters or individual transmitters.
Various technologies have been developed over time to allow different parts of the EM spectrum to be utilized by more devices simultaneously. While early applications of EM communication (such as radio and television broadcasts) assumed by default that the transmitter would be omnidirectional (so it could reach listeners in every direction) and the receiver would be omnidirectional (so it would be cheap and simple to use), and that the transmission of information from the originating transmitter to the final receiver would take place in one step, setups like that severely limit the use of the EM spectrum as compared with what is possible when other techniques (such as directional antennas and/or repeaters) are employed.
Many of the challenges that exist in the efficient utilization of the EM spectrum (both across frequency and across geographic space) have analogs in acoustics. Many people trying to talk to each other in a restaurant at the same time produces an environment with significant background noise. People at first try to compensate by talking louder. With everyone talking louder, the background noise gets even louder. Finally, people have to lean closer to each other to talk. In the end, people are leaning closer as well as shouting, whereas if everyone had been leaning closer to begin with, the shouting would not have been necessary. Perhaps one reason governments regulate EM transmission power is to avoid the same “escalation” in the EM domain, and drive technology toward more efficient solutions for point-to-point communication.
The shape of the human head and the placement of the ears allow a person to listen directionally and pick out one of many nearby conversations. Analogously, the utilization of directional receivers and/or transmitters has allowed better utilization of the EM spectrum. For example, a modern cell phone tower can broadcast to several cell phones simultaneously on the same channel, in different physical directions, and the direction of each communication can be varied over time as the people using cell phones move around (typically on foot or in cars). Likewise, satellite receivers may be pointed individually at any one of a number of orbiting satellites operating in the same frequency band.
In the cell phone application, the transmit and receive patterns of the cell phone tower antenna are highly directional and varied under computer control, while the transmit and receive patterns of the consumer's cell phone are omnidirectional (so the consumer doesn't have to know where the tower is or point the cell phone at the tower). The directionality of the cell tower antenna not only allows the tower to communicate with more cell phones simultaneously in a given portion of the EM spectrum, it also allows the transmitters in the cell phones to operate at lower power, because the directionality results in an increase in received signal-to-noise ratio when the cell tower antenna is operating as a receiver. This increase in signal-to-noise ratio is sometimes referred to as antenna “gain”. In transmit mode, because the directionality of the antenna concentrates the transmitted RF power in a particular direction, the signal intensity in that direction is effectively amplified. In receive mode, although directionality does not result in an increase in received signal, it is effectively a gain (in signal-to-noise ratio) because the antenna directionality results in a reduction in noise.
As the EM spectrum becomes more heavily utilized, more and more EM “noise” is present in our environment. Any EM signals that come from transmitters other than the one we are trying to receive from shall in this document be referred to as noise. In addition to noise, in an urban environment, for example, where metallic objects may reflect EM transmissions, the problem of “multi-path” must also be dealt with. Multi-path occurs when two versions of the same signal arrive at a receiver through pathways of different lengths. If the difference in lengths of the two paths is short compared to the EM wavelength of the highest frequency information which is modulated onto the carrier, but long enough to represent at least a significant fraction of the wavelength of the carrier itself, then multi-path can result in destructive interference at the carrier level. The probability density function in FIG. 9 illustrates the relative likelihoods that two waves arriving at an antenna with equal field strength and randomly aligned phase would sum to a composite field strength between zero and two. If one of the two pathways involves reflection of the EM signal off a moving object, loss of signal (caused by destructive interference) may come and go over time.
If the difference in lengths of the two paths is long compared to the EM wavelength of the highest frequency information which is modulated onto the carrier (such condition shall herein be referred to as Long Multi-path), ghosting of the demodulated signal will occur, such that the actual demodulated signal comprises two time-shifted versions of the intended demodulated signal (where the two time-shifted components usually also have different amplitudes). The effect of such multi-path is commonly observable as “ghost” image artifacts in broadcast TV images received in urban environments.
Making the receiving antenna highly directional significantly reduces most sources of multi-path, since in most cases the EM signals that arrive at the receiving antenna do not wind up coming from the same direction. Directional receiving antennas can be a practical solution to improving broadcast TV reception (witness the availability of roof-top TV antennas and associated servo-mechanisms to rotate such antennas under remote control), but as illustrated in the cell phone example, directional receiving antennas may not be a practical solution in an application where either the transmitter or the receiver is mobile.
Different technologies have been developed to deal with Long Multi-path in the signals received on the omnidirectional antennas of cell phones and TV sets. One technique used in some TV sets involves subtracting an amplitude-adjusted version of demodulated signal from the demodulated signal, such that the ghost phenomenon is eliminated to first order. This is done by passing the composite received signal through a Finite Impulse Response (FIR) filter with dynamically adjustable coefficients.
Another technique (used in cell phones) involves shifting the received RF signal down to an intermediate frequency (IF), and then sampling the IF and using a multi-tapped FIR filter (sometimes referred to as a “rake filter”) to effectively constructively align the arrival times of the various multi-path signals. This is usually done as part of the overall Digital Signal Processing (DSP) performed in the cell phone. Self-adjusting DSP algorithms have been developed whereby cell phones monitor and dynamically compensate out the effects of Long Multi-path interference.
It has already been mentioned that highly directional antennas comprise one method for reducing the amount of power needed to transmit over a given distance from a transmitter to an intended receiver (thus reducing EM “pollution” or noise at unintended receivers). Another method of reducing the required amount of transmit power is to utilize repeaters. In the acoustic analog of the crowded restaurant with many conversations going on, one might think of two ways of communicating with a person on the other side of the room. One way would be to stand up and yell, and another way would be to ask a series of people to pass a verbal message along until the message reaches the intended recipient.
The power needed to produce a given field strength at a given distance in the far field of an omnidirectional transmitter grows with the square of the distance. Thus, dividing the distance the signal is to be transmitted into co-linear sequential segments reduces not only the power required at each sequential (repeater) transmitter, but also reduces the summed total power of the sequence of transmitters. That is, the summed total power of the sequence of transmitters is less than the power needed to transmit the signal the entire distance using a single transmitter. Repeaters have long been used to reduce the power needed to transmit communications signals from remote areas. Repeaters can also be used to transmit “around” obstructions. For instance, a series of repeaters can be used to transmit a line-of-sight EM signal over or around a mountain.
A series of repeaters may be considered to be a multiple-discreet-element wave guide arranged in space. The series of repeaters guides a signal along a path in a way analogous to a wire or a fiber-optic cable guiding an EM signal along a path, by concentrating the propagation of that signal in a volume of space along the path, rather than having the signal propagate equally in all directions. In a military application, it may be desirable to use a series of repeaters to route a transmitted signal around an enemy, such that at the location of the enemy, the transmitted signal is too weak to receive.
While utilization of dedicated repeaters certainly aids in efficient point-to-point transmission of EM signals, this solution is not without its own drawbacks. Such drawbacks include the cost incurred to manufacture, geographically locate, and maintain an entire series of transceivers, rather than just two. In a cell-phone-to-cell-phone conversation, cell phone towers essentially act as ground-linked repeaters for passing along information transmitted from one cell phone to another. Thus while the cost in terms of total EM transmit power is lower, the cost in terms of producing and maintaining equipment may be high.
Most cell phone users are familiar with certain geographic areas where cell phone coverage “drops out”. Usually at times of highest system utilizations (such as morning and evening commuting times), the drop-out zones become larger and more frequent.
Indeed, in both civilian and military applications, it is often true that the times when more ground-based repeaters and more ground-based-repeater capacity is most needed are at times of highest system utilization. In disaster situations the need for more capacity becomes particularly acute. These situations include “acts of god” such as earthquakes or fires in places such as California, hurricanes in places such as the southeastern states, as well as situations such as the terrorist attacks of Sep. 11, 2001 during which the cellular phone system became so overloaded it was virtually useless to emergency personnel.
In view of the foregoing, a need clearly exists for self-configuring communications systems that utilize their own dynamically shifting matrix of receive and transmit nodes to route wireless communication signals in areas where no ground stations have been set up. This need exists both for military applications and for civilian applications. Such a military communication system should utilize the entire array of transceivers carried by military personally in a combat operation as nodes on a dynamically configurable repeater system. Such a civilian communication system is needed, for instance, to automatically fill in “holes” in cellular coverage (where tower antennas provide inadequate coverage) by routing calls through other cell phones (which are equipped with the present invention).